The present invention relates to a series of polymeric photoinitiators which may be used in various radiation curable compositions, including varnishes, lacquers, printing inks, coating compositions and the like. The invention also provides for radiation curable compositions which include at least one of the compounds of the present invention as a photoinitiator. Photoinitiators used in inks and varnish formulations need to have a good cure speed, and particularly good surface curing activity, low odour and good solubility. Moreover, as consumers become increasingly wary of extraneous compounds in foodstuffs, in order to comply with current and likely future legislation, the tendency of the compounds to migrate and be extracted should also be low.
At present, benzophenone is widely used as photoinitiator for ultraviolet cured inks and overprint varnishes as it has good surface curing, low yellowing and good solubility. It is also very cheap and widely available. However, benzophenone is also known for its relatively strong odour and its exceptional ability to migrate and be extracted from print into foodstuffs, even through barrier packaging such as board and plastic wrappers. Commonly used benzophenone alternatives for low odour applications include benzophenone-2-methyl ester (Speedcure MBB from Lambson) and acrylated benzophenone (EBECRYL® P38 from ALLNEX), although both of these materials are less efficient than benzophenone. Phenylbenzophenone and diphenoxybenzophenones are particularly efficient, but suffer from poor solubility and, in the case of phenylbenzophenone, increased yellowing, which means that it may be used in printing inks (which contain pigments and so can be formulated to hide the yellowing) but that it cannot be used in varnishes.
It is clear that there is a need for low odour benzophenone derivatives with good reactivity, particularly good surface curing, and a limited tendency to migrate.
It is well known that polymeric benzophenone derivatives and multifunctional benzophenone derivatives will give rise to systems that have low odour and reduced tendency to migrate and be extracted. The disadvantage with many of these systems is that their reduced chromophore equivalent per gram means they have poor cure speed. When used in higher concentration to increase reactivity, they often act as plasticizer which is detrimental e.g. for the mechanical properties of the cured ink.
EP1438282B1 describes multi-functional photo-initiators that are based on the reaction of a 4-carboxyalkoxybenzophenone with a polymeric polyhydric alcohol.
There is a need for further polymeric photoinitiators with a high functionality, good solubility in coating formulations, high reactivity, and which give rise to cured coatings which have extremely low odour, and are likely to have a much lower tendency to migrate and be extracted than most benzophenone alternatives.
We have now discovered a series of polymeric compounds based on 2-(4-phenylbenzoyl)benzoic acid as a starting material which meet these requirements.